1. Field of the Invention
This invention relates to force limiting assemblies such as torque limiters or tolerance rings, wherein the force limiter provides an interference fit between parts of an assembly.
2. Summary of the Prior Art
Force limiters limit the maximum amount of force than can be transmitted between parts of an assembly. Commonly, a first part of the assembly has a cylindrical portion located in a cylindrical bore of a second part. The invention particularly relates to assemblies having a force limiter that provides an interference fit between a cylindrical component such as a shaft or a bearing and a housing for the shaft.
Very close tolerances are required where press fits, splines, pins or keyways are employed to transmit forces in applications such as pulleys, flywheels or driveshafts.
Force limiters may be used to provide an interference fit between parts required to transmit torque or parts required to transmit linear forces. Force limiters such as tolerance rings provide a low cost means of providing an interference fit between parts that may not be machined to exact dimensions. Tolerance rings have a number of other potential advantages, such as compensating for different linear coefficients of expansion between the parts, allowing rapid apparatus assembly, and durability.
A tolerance ring generally comprises a band of resilient material, for example a metal such as spring steel, the ends of which are brought together to form a ring. A strip of protrusions extend radially outwards from the ring, or radially inwards towards the centre of the ring. Usually, the protrusions are formations, possibly regular formations, such as corrugations, ridges, waves or fingers.
When the ring is located in the annular space between, for example, a shaft and a bore in a housing in which the shaft is located, the protrusions are compressed. Each protrusion acts as a spring and exerts a radial force against the shaft and the surface of the bore, providing an interference fit between the shaft and the housing. Rotation of the housing or the shaft will produce similar rotation in the other of the shaft or the housing, as torque is transmitted by the ring. Likewise, linear movement of the housing or the shaft will produce similar linear movement in the other of the shaft or the housing, as the linear force is transmitted by the ring. If forces (rotational or linear) are applied to one or both of the shaft and the housing, such that the resultant force between the mating components is higher than a certain threshold value, the shaft or housing will move relative to one another, i.e. they will slip. In this application, this threshold value is referred to as the “slip force” of the shaft, housing and torque limiter apparatus.
Typically, the band of protrusions is axially flanked by annular regions of the ring that have no formations (known in the art as “unformed regions” of the tolerance ring).
Although tolerance rings usually comprise a strip of resilient material that is curved to allow the easy formation of a ring by overlapping the ends of the strip, they may also be manufactured as an annular band.
The term “shaft” as used hereafter includes any assembly component with a generally cylindrical portion such as a shaft or a bearing.